In a message dated 23/11/2009, rsparks@.......... writes:

There  seems to be an ongoing difference of opinion of what constitutes a  
velocity detector or an acceleration detector.  To my mind, if a  
seismometer automatically returns to a zero position, then it must be  
recording acceleration.  All vertical seismometers do this because  they 
all are measuring against gravity, so they would all be acceleration  
devices.  Excepting the sensitivity to tilt, all horizontal  seismometers 
also return to a zero position so they could also be called  acceleration 
sensitive devices.  But maybe this description  oversimplifies the 
situation.
Hi Roger,
 
    Sorry, but no. You seem to be confusing  devices which have AC and DC 
characteristics.

Some  sensors (capacitive and optical for example) clearly record 
displacement  (neither acceleration or velocity).  
    Agreed.

On the  other hand, with magnetic/coil devices, velocity is always observed 
when  electrical 
output is observed so magnetic/coil devices are velocity  detectors.
    No. They may be either velocity or acceleration  detectors. If the coil 
is attached to a ~freely suspended, but damped, mass, you  get a velocity 
detector - eg a Lehman. If it is attached to a mass suspended on  a spring, 
you get an acceleration detector. The length of the spring is constant  if 
the velocity is constant. It only changes in length and causes the coil to  
move if there is an acceleration.

A  displacement sensor would record the relative distance from a some zero  
point at the instant of data read, the velocity sensor would record the  
relative velocity at the instant of data read,  and acceleration  sensor 
would 
    detect the acceleration! eg a MEMs  acclerometer.

be a  calculated number found by using the data from any two velocity 
data  points and any three displacement data points.
    Agreed.

Finally,  if two seismometers, identical except for detectors, were 
placed side by  side, they would both plot the identical earthquake wave 
form, assuming  that the frequency characteristics were the same. 
    No, they wouldn't. A velocity output is  proportional to the 
differential (slope) of a position output plot with  time.

However, when it is recognized that displacement position is  not time 
sensitive but velocity is, the builder can expect dramatic  frequency 
response differences between displacement detectors and velocity  
detectors .  Simply put, distance is distance, but velocity is the  
distance divided by the time needed to travel between two  points.   As a 
result, for velocity detectors, the longer the  wave length, the less 
energy for each instant resulting in decreased  voltage (and current) 
detected at each instant (for any defined magnetic  field).
    Regards,
 
    Chris Chapman








In a message dated 23/11/2009, rsparks@.......... writes:
There=20
  seems to be an ongoing difference of opinion of what constitutes a=20
  
velocity detector or an acceleration detector.  To my mind, if=
 a=20
  
seismometer automatically returns to a zero position, then it must=
 be=20
  
recording acceleration.  All vertical seismometers do this beca=
use=20
  they 
all are measuring against gravity, so they would all be acceler=
ation=20
  
devices.  Excepting the sensitivity to tilt, all horizontal=20
  seismometers 
also return to a zero position so they could also be ca=
lled=20
  acceleration 
sensitive devices.  But maybe this description=20
  oversimplifies the situation.
Hi Roger,
 
    Sorry, but no. You seem to be confusing=
=20
devices which have AC and DC characteristics.
Some=20
  sensors (capacitive and optical for example) clearly record 
displace=
ment=20
  (neither acceleration or velocity).  
    Agreed.
On the=20
  other hand, with magnetic/coil devices, velocity is always observed when=
=20
  electrical 
output is observed so magnetic/coil devices are velocity=
=20
  detectors.
    No. They may be either velocity or accelerati=
on=20
detectors. If the coil is attached to a ~freely suspended, but damped, mas=
s, you=20
get a velocity detector - eg a Lehman. If it is attached to a mass suspend=
ed on=20
a spring, you get an acceleration detector. The length of the spring is co=
nstant=20
if the velocity is constant. It only changes in length and causes the coil=
 to=20
move if there is an acceleration.
A=20
  displacement sensor would record the relative distance from a some zero=
=20
  
point at the instant of data read, the velocity sensor would record=
 the=20
  
relative velocity at the instant of data read,  and acceleratio=
n=20
  sensor would 
    detect the acceleration! eg a MEMs=20
acclerometer.
be a=20
  calculated number found by using the data from any two velocity 
data=
=20
  points and any three displacement data points.
    Agreed.
Finally,=20
  if two seismometers, identical except for detectors, were 
placed sid=
e by=20
  side, they would both plot the identical earthquake wave 
form, assum=
ing=20
  that the frequency characteristics were the same. 
    No, they wouldn't. A velocity output is=20
proportional to the differential (slope) of a position output plot with=20
time.
 However, when it is recognized that displacement posi=
tion is=20
  not time 
sensitive but velocity is, the builder can expect dramatic=
=20
  frequency 
response differences between displacement detectors and ve=
locity=20
  
detectors .  Simply put, distance is distance, but velocity is=
 the=20
  
distance divided by the time needed to travel between two=20
  points.   As a 
result, for velocity detectors, the longer=
 the=20
  wave length, the less 
energy for each instant resulting in decreased=
=20
  voltage (and current) 
detected at each instant (for any defined magn=
etic=20
  field).
    Regards,
 
    Chris Chapman